This invention relates generally to well logging methods and apparatus for investigating subsurface earth formations traversed by a borehole and, more specifically, relates to methods and apparatus for evaluating fractures resulting from multiple stage fracturing of earth formations surrounding a borehole.
The concept of fracturing or formation breakdown has been recognized by the oil industry for many years. Fracturing is useful to overcome wellbore damage, to create deep-penetrating reservoir fractures to improve productivity of a well, to aid in secondary recovery operations, and to assist in the injection or disposal of brine and industrial waste material. The techniques of formation fracturing include injecting under pressure into a well bore a fracture fluid and igniting high explosives within the well bore.
Hydraulic fracturing consists essentially in breaking down a producing section of subsurface formation by the application of a fracture fluid under high pressure into the well bore. The composition of the fracture fluid is varied and can include water, acid, cement and oil. Dissolved in the fracture fluid is a material which invades the fractures created by the pressure application and serves to prevent them from closing again when the pressure is released.
Advances in the field of fracturing have yielded several multiple fracturing procedures. One form of multiple fracturing consists in submitting a single production zone to several repeated fracturing operations. The purpose is to further extend formation fractures created by the previous fracturing operation to provide thorough fracturing of a producing zone.
Another multiple fracturing procedure has been devised wherein several formation zones are fractured by subjecting them to successively higher fluid pressures. After the first pressure application, the fractures formed are temporarily sealed at the wall of the well with a chemical reagent carrying suspended solids. The pressure is then increased until a new set of fractures forms in a different formation zone. The procedure is repeated a numbers of times, after which the sealing agent is liquified by chemical treatment, thus opening all of the fractures and leaving multiple formation zones fractured.
It is desirable to evaluate each of the several successive fracture treatments to determine the degree of fracturing created by each treatment and to determine which formation zone was fractured by any one particular treatment. Previously, the evaluation consisted of logging the formations surrounding the borehole after each of the successive fracturing treatments. The logging operation can involve one of several recognized logging instruments including the induction log, dip meter and variable density acoustic log. Such successive logging operations are costly in loss of time involved and in the expenditure of mony required for the service.
These and other disadvantages are overcome with the present invention by providing methods and apparatus for evaluating multiple stage fracturing treatments in a single logging operation conducted after the completion of the entire multiple fracturing operation.